1. Field of Invention
This invention is directed to a circuit that can be both non-permanently programmed and permanently programmed.
2. Description of Related Art
Fuses and fusible links are circuit elements that open by burning out or breaking when a relatively high current is applied. By selectively breaking or leaving intact specific fusible links, a circuit can be customized or programmed using these fusible elements. An antifuse is the opposite of a regular fuse. That is, an antifuse is normally an open circuit until a programming current is forced through it. Fuses and antifuses may be used to address many problems, including calibration requirements of analog circuits such as digital/analog converters, or current or voltage sources, logic synthesis circuits such as digital delays lines, or chip specific performance data to be used by the end system in which the chip will be used. These fusible and antifusible circuits are generally xe2x80x9cprogrammedxe2x80x9d after chip fabrication has been completed and during the wafer testing phase of chip production. The programming may be used to add additional resistors into a circuit to compensate for variations introduced during the manufacturing process or to compensate for oscillator frequency variations induced by manufacturing stress.
In many of these cases, it is desirable to simulate a programmed state before actually programming the device. For example, analog circuit calibration may require additional steps of simulation and refinement based on the previewed or simulated results obtained. These results are incorporated into further simulations to correctly calibrate the circuit during the testing phase. After testing, the circuit may then be permanently programmed.
Conventional programmed circuits employing, for example, fuses and anti-fuses, generally do not permit preview or simulation of the programmed circuit. Conventional programmed circuits require specialized packaging to ensure no overlay of the fusible or antifusible link occurs. That is, a conventional circuit can only be permanently programmed and not previewed. Once the conventional circuit is programmed, no further changes or refinements to the circuit are possible.
U.S. Pat. No. 6,037,871 to Watrobski et al. describes such a fusible link circuit including a preview feature that uses fusible links in combination with transistors to permanently set the value of an output. However, this fusible link circuit also requires special manufacturing and packaging techniques, as discussed above. For previewable devices such as those described in Watrobski, the device imposes specialized packaging, manufacturing, handling and cost limitations. Conventional fuse programming methods require that the selected device packaging technique be suitable for the programming structures. For example, the device packaging techniques need to facilitate air access to the fuse for burning and to avoid overlays that may act as heat sinks. A heat sink would increase the fuse blow temperature, which could possibly exceed the circuit temperature tolerance. Thus, manufacturers of products requiring these features must select packaging and fabrication techniques that are tailored to these fusible circuits and which are typically more expensive than non-programmable circuit fabrication and packaging techniques.
Erasable programmable read only memory (EPROM) and electrically erasable programmable read only memory, (EEPROM), devices may be programmed and reprogrammed. However, EPROM devices rely on specialized fabrication techniques and typically include a quartz window through which ultraviolet light of a specific wavelength may be introduced for several minutes to erase the chip in preparation for re-programming. In use, the quartz window is covered to prevent accidental erasure of the device. The EPROM devices require physical removal of the chip and or physical manipulation of the cover over the quartz window as well as considerable time to effect erasure in preparation for re-programming. Thus, EPROM devices require both specialized fabrication techniques and specialized handling during programming.
EEPROM circuits typically use floating gates surrounded by a much thinner insulating layer which can be erased by applying a voltage of the opposite polarity to the charging voltage to the non-floating gate. EPROM circuits overcome some of the EPROM device limitations with respect to the use of ultraviolet light to effect erasure. However, EEPROM devices also require special fabrication techniques in their manufacture. Furthermore, EEPROM devices require that special opposite polarity voltage levels be adopted for reading and writing to the EEPROM device.
Conventional programmable circuits are either write-once circuits that can be programmed but not re-programmed or read and write devices that can be programmed and re-programmed. The write-once circuits do not provide for simulating or previewing the state of a circuit before programming the circuit permanently. The read and write devices can be re-programmed after simulating or previewing the circuit but require special handling and manufacturing techniques.
Thus, a programmable circuit is needed that can be fabricated using conventional reliable and inexpensive fabrication techniques and that provides a preview function which uses normal circuit voltage levels for permanent programming and higher voltage levels for permanent programming.
This invention provides systems and methods for programming a circuit using a pull-down transistor as an antifuse in the circuit.
This invention separately provides a circuit having a preview function useable to simulate circuit characteristics using normal voltage.
This invention further provides a circuit that uses a higher voltage to permanently program the circuit.
The circuit can be fabricated using conventional, inexpensive and reliable fabrication techniques. The circuit includes a spike enable input structure which is used to determine when the chip is to be programmed. The circuit also includes a test and spike input structure which has at least two functions controlled by the setting of the spike enable input structure.
When the spike enable input structure is in the default or open state, a programmable transistor is in an open state. As a result the voltage at the output structure is at a first predetermined value. External tester electronics can be attached to the test and spike input structure to drive the output structure to the first predetermined voltage by leaving the test and spike input undriven or by driving the test and spike input structure to the first predetermined voltage. Conversely, the external electronics can drive the output structure to a second predetermined value by driving the test and spike input structure to the second predetermined value. Thus, the output of the circuit can be simulated or previewed before permanent programming of the circuit occurs.
A programmed state of the simulated programmable previewable circuit can be created by asserting an enable signal on the spike enable input. A test and spike input is then pulsed with a programming voltage that is higher than either the first or second predetermined voltages. The programming voltage is of sufficient amplitude, pulse width and frequency to cause the programmable transistor to enter the failure mode, known as snap back. During the snap back mode, the programmable transistor carries a large current density. The large current density in turn causes silicon to dissolve into the metallic layer connected to a source drain of the programmable transistor, so that a void is created below the interface which is then infilled by metal flow in a process called spiking. The metal tends to form sharp spikes which penetrate the junction and forms a short circuit which creates a permanent closed path. Thus, a permanent closed state is created by spiking the programmable transistor as a result of the programming operation. The circuit may still be changed and previewed during the non-permanent default, or simulation phase.
Various exemplary embodiments of the methods according to this invention comprises applying a signal having the second predetermined voltage to the test and spike input structure while a signal is applied to the spike enable input structure to hold the programmable transistor in an open state. The output signal generated in response to the applied input signal is compared to the desired output signal. A determination is made whether the generated output signal corresponds to the desired output signal. If the comparison indicates that the examined output signal corresponds to the desired output signal, a programming signal is then applied to the test and spike input while the spike enable input is driven to the closed state to permanently program the pull-down transistor to a spiked condition, thus, the second predetermined voltage becomes permanently applied to the output structure.
It should be noted that the programmable previewable circuit of this invention does not rely on fuses. Instead, the programmable previewable circuit uses spiking of the transistor to permanently set the circuit to a closed state. Therefore, the programmable previewable circuit can be used in circuits without the special packaging normally required to minimize heat sink problems. Since the circuit can use conventional, inexpensive and reliable manufacturing techniques, it will reduce or eliminate the manufacturing changes required to use conventional programmable circuits.
This is a major advantage in situations where the integrated circuit must be covered by some passivation or encapsulation since any material covering a fuse will act as a heat sink requiring higher currents and voltages in order to cause the fuse to blow. These higher currents and voltages could damage the surrounding circuits.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments of the systems and methods according to this invention.